This application claims the priority of Application No. 2001-53428, filed Feb. 28, 2001, in Japan, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an electric motor which uses a permanent magnet as a field magnet, and particularly to an electric motor and a control method by which the machine tool is driven or an electric power is generated.
In a permanent magnet type electric motor of the prior art, an induced electromotive force E is determined by a constant magnetic flux Φ generated by a permanent magnet arranged in a rotor and a rotating angular speed ω of the motor. That is, when the rotating angular speed ω (rotating speed) of the motor is increased, the induced electromotive force is proportionally increased.
Accordingly, high torque can be obtained in a low speed range, but the output voltage of the inverter is limited because the induced electromotive force increases as the number of revolution increases. As a result, the operation Therefore, the high-speed operation range is widened using a field weakening control technology.
Moreover, the main shaft motor of a machine tool is provided with a gear mechanism to maintain the fixed output in a wide speed range to deal with various conditions of working. Recently, such a motor is driven after the winding for each phase of the main shaft motor is changed-over between the winding for low speed and the winding for high speed according to the rotational speed of the main shaft by using a winding change-over unit.
Although it is possible to extend a high speed drive range by using the field weakening control described in the prior art, that is, by using the magnetic flux weakening current (d-shaft current of an armature) in a permanent magnet type electric motor, there is a limit because of the heat generation due to the current or the decrease in efficiency.
Moreover, although it is desirable to rotate the motor at high speed when material hard to cut like aluminum alloy is worked, there is a limit because of the increase in the induced electromotive force of the permanent magnet.
When the gear mechanism, etc. are provided to maintain the predetermined output power in the wide speed range, the number of mechanical parts increases, and the problems on noise or mechanical vibration occurs.
When a winding change-over device for changing-over the winding of each phase according to the rotational speed of the main shaft, the number of lead wires from the main body of the electric motor increases, and the configuration of the winding change-over device becomes complicated.